In U.S. application Ser. No. 667,306 (filed Mar. 16, 1976), now U.S. Pat. No. 4,242,661, a device for recording a first type of objects, so-called recording objects, is shown which moves relative to a second type of objects, called recording places. The recording objects, for example, may be vehicles, and the recording places may be placed along a road section passed by the vehicles. The device comprises a number of recording units mounted one in each recording place or on each recording object and a number of recording transducers one on each recording object and, respectively, in each recording place. Each recording unit comprises a transmitter and a receiver with an indication unit, and each recording transducer includes a frequency converter for converting a first signal received from said transmitter into a second signal without supplying new energy to the signal, which second signal is frequency-shifted by a certain amount relative to the first signal, and a coding means to provide said second signal with a code individual for the recording transducer, and a transmitter means for transmitting said second coded signal.
The receiver of the recording unit is arranged to receive said second signal, and its indication means is arranged to indicate said code. The frequency converter includes a phase modulator, which modulates the first signal in such a manner that at least one sideband or side frequency is formed. The coding means is adapted to provide the sideband or side frequency with said code individual for the recording transducer, preferably by starting and stopping said modulator according to a pattern constituting said code.